Here you can find the XAML style of a LineSeries DataPoint and by default the tooltip is defined as
<ToolTipService.ToolTip>
<ContentControl Content="{TemplateBinding FormattedDependentValue}"/>
</ToolTipService.ToolTip>
so, you can take the whole style to your application and override the tooltip definition for example:
<ToolTipService.ToolTip>
<StackPanel Margin="2,2,2,2">
<ContentControl Content="{TemplateBinding IndependentValue}" FontSize="12"/>
<ContentControl Content="{TemplateBinding DependentValue}" FontSize="12"/>
</StackPanel>
</ToolTipService.ToolTip>
and you willget displayed the "X, Y" tooltip you need.